Generally, differential pressure transducers contain a displacement sensor coupled between two thin diaphragms. The two diaphragm arrangement performs a mechanical subtraction of pressures applied to the diaphragms. The sensor measures the net motion of the diaphragms relative to the transducer body to determine the differential pressure. In order to prevent diaphragm rupture while maintaining the desired sensitivity to differential pressure, the volume between the diaphragms, which includes the sensor, is filled with a hydraulic fill fluid. When process line pressure is presented to one side of each diaphragm, the fill fluid is pressurized to the line pressure. If the boundary of the volume between the diaphragms, including the diaphragms themselves, the electrical feed-throughs, and fill/bleed ports, are each not properly sealed, small leaks of fill fluid will occur. This will cause unacceptable increases in response time, sensor output drift, and transducer non-linearity with pressure. In some cases, these changes may not be readily detected when the transducer is in service because the transducer output may remain stable at constant differential pressures. The leaking of fill fluid from these known differential pressure transducers is a problem that is well known and documented.
Another deficiency in fluid-filled differential pressure transducers is a static pressure effect. A differential pressure transducer as described should output a value of zero when the same process pressure is applied to both diaphragms. However, the static pressure causes the fill fluid to be pressurized, which resulting in distortions in the transducer body. These distortions cause relative motion between the diaphragms and transducer body resulting in a static pressure effect. This effect causes values other than zero when both diaphragms experience equal process pressures. The distortions also produce radial forces on the diaphragms, which change the effective stiffness of the diaphragm and causes static pressure effects on span. In addition, the displacement sensor is exposed to the fill-fluid pressure environment adding to the static pressure effects on both zero and span. In applications involving static pressures of several thousand pounds per square inch (psi) or greater, the requirement for a stable zero and span over the allowable range of static pressure is difficult to achieve in practice.
Yet another deficiency in fluid-filled differential pressure transducers is the effect of hydrogen. When differential pressure transducers are operated in a hydrogen-rich environment, for example, in a hydrocarbon processing facility, the hydrogen gas easily diffuses through the diaphragms and into the fill fluid. For example, if the differential pressure transducer is used to measure pressure differences in a hydrogen-rich high-pressure pipeline, the fill fluid will experience the large static pipeline pressure and hydrogen will diffuse through the diaphragms into the fill fluid. When the pipeline pressure is reduced, such as during scheduled shutdowns, hydrogen gas boils out of the fill fluid and forms bubbles. Since the enclosed volume of fill fluid is constant, bubbles of hydrogen within the closed volume deform the diaphragms, resulting in a calibration shift, zero offset, or in the worst case, diaphragm rupture.
The use of a fill fluid also contributes to degraded performance of differential pressure transducers when operated over a range of temperatures, as is normal in service. The volumetric expansion of liquids with changes in temperature is significantly greater than that of the metals used in construction of the transducer body. Thus, when the environmental temperature of either the differential pressure transducer or process fluids changes, the volume of the fill fluid in the transducer and capillary lines changes. Unless the changes in fill-fluid volume are perfectly balanced on both the high and low-pressure sides of the transducer, the result is significant errors in the output of the transducer. The normal method for limiting this effect is to keep the volume of the fill fluid at an absolute minimum. However, in addition to only limiting the problem and not eliminating it, this method aggravates the effect of fill fluid leakage because any loss of fluid is a more significant part of the total fluid volume.
Rather than perform a mechanical subtraction of two large pressures as described above, an alternative approach is to measure each pressure with separate gage pressure transducers and perform an electronic subtraction to calculate the differential pressure. If the full-scale differential pressure range to be measured is 400 inch H2O (15 psi) and the desired accuracy is 0.1% of the full scale range (i.e., 0.015 psi), then for applications at 3000 pounds per square inch gauge (psig) line pressure, a gage pressure transducer is required that has an accuracy of 0.015/3000=0.0005% (1:200,000). Such devices are not commercially available. Thus, electronic subtraction is not practical and mechanical subtraction of two large pressures is the only practical alternative measurement approach available with present day technology.
Presently, there is no known system or method for providing a differential pressure transducer that avoids the problems associated with the known devices listed above. The present invention as described and claimed herein, addresses the deficiencies of prior art differential pressure transducers